Mycoplasmas are widely distributed in nature and commonly produce diseases of considerable economic impact, yet little information is available concerning mechanisms of pathogenesis and effective methods of control are unavailable. Many mycoplasmas undergo rapid variations in surface proteins that are thought to be important to disease pathogenesis. The issue of whether variations in these mycoplasmal proteins is primarily a mechanism for immune avoidance or instead a mechanism for creating cells with varied functions (e.g., subpopulations of cells may produce different adhesions leading to tissue tropism) has not been addressed. In the murine pathogen Mycoplasma pulmonis, high-frequency phenotypic variations involving changes in the highly repetitive V-1 surface antigens affect colony morphology, the susceptibility of the organism to mycoplasma viruses, the adsorption of mycoplasmas to red blood cells, and virulence. The applicant's laboratory has recently shown that the V-1 antigens are encoded by a family of genes designated vsa (variable surface antigen). The portion of the vsa expression locus (including the promoter and first 712 nucleotides of the coding region) is present as a single copy in the chromosome. Most vsa genes lack expression signals and are silent; only one vsa gene is expressed in any given cell. DNA rearrangements occur in the vsa locus at a high frequency and alter vsa gene expression by combining the 5' end from the expressed gene with the 3' end of a previously silent gene. The rearrangements characterized thus far are DNA inversions that occur within a specific 34-bp sequence that is present in all vsa genes. This sequence is referred to as the vrs (vsa recombination site) box. The applicant's long-range goals are to (i) clone the entire vsa locus and determine its complete nucleotide sequence, thereby identifying the full repertoire of vsa genes and associated vrs boxes, (ii) examine more closely the association between DNA rearrangements and changes in vsa gene expression by characterizing high-frequency DNA rearrangements that occur within vsa, (iii) elucidate the enxymatic mechanis(s) responsible for vsa rearrangements by constructing recombination- deficient mutants, and (iv) explore the association between vsa gene rearrangements and disease pathogenesis by evaluating recombinase-deficient mutants in animal models.